Conventionally, HE-AAC (High Efficiency MPEG (Moving Picture Experts Group) 4 AAC (Advanced Audio Coding)) (International Standard ISO/IEC 14496-3), etc. are known as audio signal coding techniques. With such coding techniques, a high-range characteristics coding technology called SBR (Spectral Band Replication) is used (for example, see PTL 1).
With SBR, when coding an audio signal, coded low-range components of the audio signal (hereinafter designated a low-range signal, that is, a low-frequency range signal) are output together with SBR information for generating high-range components of the audio signal (hereinafter designated a high-range signal, that is, a high-frequency range signal). With a decoding apparatus, the coded low-range signal is decoded, while in addition, the low-range signal obtained by decoding and SBR information is used to generate a high-range signal, and an audio signal consisting of the low-range signal and the high-range signal is obtained.
More specifically, assume that the low-range signal SL1 illustrated in FIG. 1 is obtained by decoding, for example. Herein, in FIG. 1, the horizontal axis indicates frequency, and the vertical axis indicates energy of respective frequencies of an audio signal. Also, the vertical broken lines in the drawing represent scalefactor band boundaries. Scalefactor bands are bands that plurally bundle sub-bands of a given bandwidth, i.e. the resolution of a QMF (Quadrature Minor Filter) analysis filter.
In FIG. 1, a band consisting of the seven consecutive scalefactor bands on the right side of the drawing of the low-range signal SL1 is taken to be the high range. High-range scalefactor band energies E11 to E17 are obtained for each of the scalefactor bands on the high-range side by decoding SBR information.
Additionally, the low-range signal SL1 and the high-range scalefactor band energies are used, and a high-range signal for each scalefactor band is generated. For example, in the case where a high-range signal for the scalefactor band Bobj is generated, components of the scalefactor band Borg from out of the low-range signal SL1 are frequency-shifted to the band of the scalefactor band Bobj. The signal obtained by the frequency shift is gain-adjusted and taken to be a high-range signal. At this time, gain adjustment is conducted such that the average energy of the signal obtained by the frequency shift becomes the same magnitude as the high-range scalefactor band energy E13 in the scalefactor band Bobj.
According to such processing, the high-range signal SH1 illustrated in FIG. 2 is generated as the scalefactor band Bobj component. Herein, in FIG. 2, identical reference signs are given to portions corresponding to the case in FIG. 1, and description thereof is omitted or reduced.
In this way, at the audio signal decoding side, a low-range signal and SBR information is used to generate high-range components not included in a coded and decoded low-range signal and expand the band, thereby making it possible to playback audio of higher audio quality.